Traffic congestion dropped 30% last year from 2010 in the USA’s 100 largest metropolitan areas, driven largely by higher gas prices and a spotty economic recovery, according to a new study by a Washington-state firm that tracks traffic flows.

That was the largest drop since the nation plunged into recession in December 2007.

Of the 100 most populous metro areas, 70 saw declines in traffic congestion while just 30 had increases, says Jim Bak, co-author of the 2011 U.S. Traffic Scorecard for Kirkland, Wash.-based INRIX.

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